LED Package-on-board Assembly: Key Issues and Impact on Reliability

A Cookson Group Company

V 2 2010

LED Package-on-Board

Assembly: Key Issues

and Impact on Reliability

Ravi M. Bhatkal, Ph.D.

Alpha LED Technologies

a Cookson Group Company A Cookson Group Company

Outline

• Background

• Package-on-board assembly overview

• Key issues and impact on reliability

– Materials selection and design

– Manufacturing process and environment

• Recommendations


Background

• In the lighting industry, surface mount technology (SMT) as applied to light source manufacturing is a relatively new process.

• SSL Assemblies need to meet high reliability requirements such as, for example Energy Star Category A and B50, L70 of 35,000 hours for outdoor lighting.

• As a result various issues in package-on-board assembly materials and processes need to be addressed along with their impact on reliability.


Background

• LED soldering is different than soldering an electronic component due to the fact that an LED is a light emitting optical component.

• Assembly materials and process must not interfere with the design and lifetime requirements of the LED package and luminaire.

• The materials and process used must be designed to take performance and lifetime requirements into consideration.

• This paper outlines issues typically encountered in the LED package-on-board assembly process and optimization of the assembly process.

• Recommendations for material selection and manufacturing process are suggested that enable high reliability Luminaire fabrication.


LED Package-on-Board Assembly

Component

Placement

Package-on-Board Assembly – Wave

Wave

Solder Inspection line Aqueous

Cleaner

Package-on-Board Assembly - SMT

Printer or

Dispenser

Component

Placement

Reflow Oven Inspection line


Printing


Printer or

Dispenser

Component

Placement



What is Printing?

• Printing is the deposition of a known amount

of solder paste on a component pad.


Factors That can Affect

the Printing Process: Stencil Design

Printer type

Board supports

Alignment

Gasketing

Solder Paste

Cleaning cycle

Printing environment

Printing parameters

Speed, pressure, cleaning


Issues in Printing for LED Assembly

Issue Cause Recommendation

Bridging between anode,

cathode and thermal pad, or

solder ball formation

Excess solder paste deposit due to

improper stencil design and printing

parameters

Proper stencil thickness and

aperture design relative to the

board construction and

component pads

Package tilt upon assembly

causing off-axis light output

Unequal heights of solder joints under

anode, cathode and thermal pad, due

to unequal or inconsistent paste

deposits.

Proper stencil design relative to

the pad geometry of the package

and use of a solder paste that has

good release characteristics, and

correct print parameters

Insufficient joint thickness

negatively affecting joint

reliability in use

A lower joint thickness due to a

thinner stencil than recommended, or

insufficient deposit will cause

reduction in the amount of solder

deposited

Proper stencil thickness selection

and aperture design, as well as

selection of paste for proper

transfer


Issues in Printing for LED Assembly


Contamination of lens

during or after assembly

Selection of a solder paste whose flux

has a low softening point relative to

the operating temperature of the

device, or has a high level of spread

during reflow, or excessive spatter,

thus resulting in migration of the

paste flux used and creating

contamination of the lens

Select a solder paste with with

appropriate chemistry

providing low residues, low spread

and low spatter,

Contamination of Board

Reflective Surface

Selection of a no-clean solder paste

with an opaque residue and high

spread or spatter

Select a solder paste with with

appropriate chemistry

providing clear residues, low

spread and low spatter,


Recommendations - Printing

• Conduct process development early, ensuring

correct stencil thickness and aperture design,

accounting for component pad geometry, type

of board and board surface features such as

solder mask defined pads, paste selected and

print parameters.


LED Package Pick

and Place


Printer or

Dispenser

Component

Placement



Issues in Pick and Place For LED Assembly


Damage to optical surface

(lens, encapsulation)

Incorrect handling of the LED

package during assembly, including

use of incorrect pick and place nozzle

Proper handling of LED which

avoids contact with lens or

encapsulation, and use of correct

nozzle

Incorrect placement of the

LED package, resulting in

misaligned or damaged

package

Misalignment of the LED package

with the solder paste deposits, or use

of excessive Z force, causing

displacement of solder paste deposits

Ensure proper alignment of solder

paste on pad and component on

solder paste, and use correct Z

force.

Misplacement of the LED

package – incorrect package

– board orientation

Misoriented package during pick-up Ensure that pick and place

equipment is capable of detecting

orientation and correcting if mis-

oriented


Recommendations – Pick and Place

• Ensure use of appropriate pick and place

equipment, pick up nozzle and programming,

enabling correct pick up and placement with

correct alignment, orientation and pressure.


Reflow Soldering

& Profiling


Printer or

Dispenser

Component

Placement



Anatomy of a Solder Reflow Profile

0

50

100

150

200

250

0 60 120 180 240 300 360 420

Time (sec.)

Tem

pe

ratu

re (

C)

Ramp Soak Reflow Cool


(1) Ramp

• Temperature rise rate to attain solvent (carrier) evaporation

Time Temperature

(1) Initial Ramp

Solvent Evaporation

Large Mass PCBs

Low Mass PCBs


(2) Preheat/Soak

• Temperature range at which the flux begins to remove oxides

and temperature deltas are allowed to stabilize/soak

• The ‘Soak Zone’ has to be long enough for components to come

to temperature, to avoid thermal shock (cracking)

– Longer for higher mass PCBs

Time

(2) Preheat

Temperature

Soak Zone Large Mass PCBs

Low Mass PCBs


(3) Reflow

• Temp spike to melt the alloy and allow the solder to wet and form the

proper fillet

– Melting point (Tmelt) of solder

• Typically >217°C for SAC solder paste

– Max Temperature achieved must be below the max temperature for

all components, including the board

– Time Above Liquidus (TAL) will largely determine the inter-metallic

growth, usually 30~60 seconds

Time Temperature

Tmelt

TAL

Tmax

Large Mass PCBs Low Mass PCBs

TAL


(4) Cooling

• Solidification of the solder joint and cooling of the laminate

– Faster cooling results in finer grain size with higher fatigue

resistance

• Cooling Zone Affects:

– Joint Strength

– Joint Appearance

Time Temperature

(4) Cooling

Large Mass PCBs

Low Mass PCBs


Issues in Solder Reflow for LED Assembly

Profile Zone Too Little Time Too Much Time

Ramp (fast ramp)

Solder Spattering

Tombstones (if no soak)

High delta T’s entering spike zone

(if no soak)

Component pop corning/cracking

(slow ramp)

Hot slump, potential MCSB’s

Fluxes spent before metal

reached liquids

Tombstones (if no soak)

Soak (fast, cool, or no soak)

High delta T’s entering spike

Little protection against

tombstones

Potentially heavier flux residues

* Recommended soak parameters

should be more like a slowed

ramp (0.5 C/sec) than a flat

plateau.

(long or hot soak)

Charred flux residues, difficult to

probe at test

Spent flux resulting in poor wetting

Random solder balls

Mid-chip solder balls

Solder bridging (will not pull back)


Issues in Solder Reflow for LED Assembly

Profile Zone Too Little Time Too Much Time

Spike (short TAL or low peak T)

Grainy-looking joints

Poor fusion

Low fillet wetting height

Minimized spread

Intermittent opens (center BGA)

(Long TAL or high peak T)

Dewetting

Charred residues

Board warpage and delamination

Component damage

Cool (Rapid cooling)

Thermal shock to components

possible (follow mfg guidelines)

Typically, the faster cooling

provides a finer grain structure,

which makes for better cosmetics

Intermittent opens (corners

PBGA)

(Slow cooling or ambient)

Extremely poor cosmetics:

-rough surface

-fillet cracking

-fillet lifting on PTH

Displaced components/disturbed

joints (when exit temp is above

solidus)


Reflow Attribute Balance

Attribute Fast or Cool Slow or Hot

MCSB Better Worse

Tombstones Worse Better

Voiding Worse Better

RSB Better Worse

Spattering Worse Better

Cosmetics Better Worse

NOTE: This speaks in general terms, does not refer to extremes, and can

vary with products formulated for specific market segments


Recommendations - Reflow

• Consider the component and material stack

while designing the reflow profile

– Leadframe based LED or ceramic submount

LED

– FR4 or MCPCB and surface finish

– Thermal mass of the board

– Solder alloy and flux (paste used)

• Conduct process development early to

determine voiding, shear strength and thermal

cycling performance


Materials Selection

Issues


Impact of Materials Stack on Reliability

• Reliability/lifetime is dependent on:

– The material stack selected (submount

/package, solder, board material and dielectric,

and respective geometries),

– The manufacturing process conditions and

– The use environment (temperature swings and

harshness of conditions)


Materials Stack Impact e.g. Al MCPCB

• Key materials stack variables – ΔCTE between the package/submount and the board

material

• E.g. ΔCTE between AlN submount and Al MCPCB is 20

– Geometry of the materials stack

• Length scale of the package/submount

– Larger package, higher stress

• Thickness of solder joint

• Thickness of dielectric

– Moduli of materials

• Moduli of submount, solder and dielectric


Process and Environment Impact

• Key process and use environment variables – Process temperature and speed of temperature

change

• Higher temperature coupled with higher ΔCTE will induce more residual stress in assembly

• Faster temperature changes will induce more residual stress

– Temperature swings in use

• Large temperature swings coupled with high ΔCTE between the package/submount and the board material will cause high stress and creep fatigue in use

a Cookson Group Company A Cookson Group Company Source: Werner Engelmaier

Lifetime of Solder Joints


Material Stack Impact on Reliability

Al2O3 CTE =7

AlN CTE = 5

Al CTE = 25

Cu CTE = 16

FR4 CTE = 17-18

Differential thermal

expansion

Stress relief via creep in solder and

strain in dielectric


Implications

• A combination of relevant material stack design,

thermal cycling tests, coupled with advanced

modeling, can provide a mapping function

between accelerated testing and estimated

lifetime in use.


Implications

• Data shows that:

– The creep resistance of the solder is a significant

factor in minimizing failures in solder joints due to

strains incurred in thermal cycling.

– The relationship between the modulus of the dielectric

to the modulus of the solder over the temperature

range in the thermal cycle can be an effective way to

manipulate the strain away from the solder joint in

thermal cycling, hence reducing failures due to solder

joint fatigue.


Materials Selection Impact on Reliability

Recommendations

• For a given use cycle (temperature swing and dwell time):

– Use a more creep resistant solder material to increase reliability of the solder joints subjected to such cycling.

– An MCPCB with a copper substrate would put less strain on the solder joint resulting in less damage to solder joints.

– Use a lower modulus dielectric to help absorb some of the strain from differential thermal expansion.

– Use an optimized combination of solder and dielectric materials in order to provide the required reliability for a given application.


Conclusions

• A combination of materials, manufacturing

process and environmental use conditions

determines reliability of LED package-on-

board assemblies and luminaires

• It is possible to optimize design, materials

selection and manufacturing process to

provide reliability required for a given end use

environment condition.


Thank You

For more information, contact:

Ravi Bhatkal

[email protected]

+1-908-791-3013

mailto:[email protected]

LED Package-on-board Assembly: Key Issues and Impact on Reliability

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